feat: add C solution to lcof2 problem: No.012 (doocs#2195)

Author: GiggleDing

lcof2/剑指 Offer II 012. 左右两边子数组的和相等/README.md

@@ -198,6 +198,29 @@ class Solution {
 }
 ```
 
+### **C**
+
+```c
+int pivotIndex(int* nums, int numsSize) {
+    int left, right;
+    left = 0;
+    right = 0;
+
+    for (int i = 0; i < numsSize; i++) {
+        right += nums[i];
+    }
+
+    for (int i = 0; i < numsSize; i++) {
+        right -= nums[i];
+        if (right == left)
+            return i;
+        left += nums[i];
+    }
+
+    return -1;
+}
+```
+
 ### **...**
 
 ```

lcof2/剑指 Offer II 012. 左右两边子数组的和相等/Solution.c

@@ -0,0 +1,18 @@
+int pivotIndex(int* nums, int numsSize) {
+    int left, right;
+    left = 0;
+    right = 0;
+
+    for (int i = 0; i < numsSize; i++) {
+        right += nums[i];
+    }
+
+    for (int i = 0; i < numsSize; i++) {
+        right -= nums[i];
+        if (right == left)
+            return i;
+        left += nums[i];
+    }
+
+    return -1;
+}